I was going to have my old Pentium-4 Dell Dimension 4500 upgraded, but apparently the motherboard can't be replaced. :/

So, I was thinking about an IBM (or Lenovo) ThinkCentre of some sort!

What I want is the fastest 32-bit Windows XP system possible!
I would like it to be fast enough to play — and edit — 1920 x 1080 HD videos as well as play the Sims 3.
As far as drives go, I would like it to at least have a DVD burner, as well as a 3½-inch floppy diskette drive.

Just so you're aware of it, in less than a years time XP be will no longer be supported, which would include no longer getting security updates. I say get a Windows 7 machine. It's optimized for SSDs, which the part most likely to make your system seem fast. It also has a XP mode if you need to run some older programs. Playing 1080 video isn't particularly taxing for most systems these days. Certainly the ThinkCentre M series is worth a look. It's just a matter of which form factor you want.

I am acutely aware of the Windows XP End-of-support situation, but thanks though.
I am not interested in Windows 7. I have given it a very fair trial. It is okay, but does not suit my needs (as well as XP.)

I am basically wondering which is the fastest ThinkCentre system that

still supports Windows XP, and

includes a floppy disk drive (or at least a slot for one, and a compatible motherboard!)

If I wanted a faster computer, couldn't I just add an SSD anyway?

I'm thinking the best form factor would the one that is designed to lay down horizontally, so you can sit your monitor on top! I don't care about the size of form factor though. Standard is fine or whatever.

I believe that would be the desktop form factor that sits a top the desk.

Adding a SSD is a little more tricky with XP. I presume you'll be installing it via a XP disc, but a XP disc won't align the partition properly for best performance. You'll need a Vista or Windows 7 disc to get proper alignment.

Hmm...
I guess I don't need an SSD drive. It'd just be nice to have, I suppose.
I suppose there's no way to manually configure them in the way that Windows 7 or Vista would.
I understand that you would normally install the operating system and all system files on the SSD and then put whatever else on a regular hard drive (depending on the size of SSD, I guess.)

Do you happen to know which was the last ThinkCentre system to support floppy disk drives, or how I could find out?

I would typically use ThinkWiki, but it only lists actual ThinkPads. :/

If you want legacy hardware support, I would recommend an IBM IntelliStation M Pro 9229 and skip the lenovo thinkstations (lenovo just outsources generic mobos like HP).
The only caveat is the fastest CPU they support is a 2.98 core2quad extreme and 8GB of DDR2 RAM. They can run windows 7 and SSDs (that's how I have mine configured) along with a native 1.44 MB FDD.

They support the following:
*360K to 1.44 MB <-- that's right, you can run a 5 1/4" 360K drive in one of these computers
*Two serial ports
*Two firewire
*One parallel
*SAS / SATA / IDE

etc etc etc

They are some of the most fully featured computers you'll ever see. Not to mention, they do a POST beep just before the FDD seek like a real computer should
The motherboard was designed internally by IBM via the System x team (they named it "Taroko").